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Research Progress Report, no. 311 (Jul. 1967)

Page 001

Research Progress Report 311 July, 1967
Optimum Wood Moisture Content Levels for the Manufacture
Of Furniture in Indiana
C. A. Eckelman and F. E. Goodrick, Department of Forestry and Conservation
Introduction
Wood is a hygroscopic material and as such will tend to adsorb or desorb moisture until it comes into equilibrium with water vapor in the atmosphere. Adsorption and desorption actually occur simultaneously --water molecules are constantly entering and leaving the surface of the wood. But if one phenomenon takes place at a faster rate than the other, a net adsorption or desorption occurs. When a point is reached at which the rate of adsorption just equals the rate of desorption, the moisture in the wood is said to be in equilibrium with that in the surrounding atmosphere. The moisture content level at which equilibrium occurs if referred to as the equilibrium moisture content (emc), where moisture content is expressed as a percentage of the oven-dry weight of the wood. The corresponding relative humidity of the surrounding atmosphere is referred to as the equilibrium relative humidity. Although many exceptions exist, the equilibrium moisture content for given relative humidity conditions is usually taken the same for all species.
The relationship between the equilibrium moisture content of wood and the corresponding relative humidity is usually expressed by means of typical equilibrium moisture content
curves. These curves may be approximated by mathematical expressions of the form
EMC = [ -ln(1 - Ø) ÷ C(T+460)]^1/n
where Ø = the relative vapor pressure
T = degrees F.
C = a constant
n = a constant
For the particular case of T = 70 degrees F this expression may be written as
EMC =[(-ln (1 - Ø)) ÷ .02345] ^ 1/1.5235
or, for any temperature between 32 and 210 degrees F. as
EMC = [ -ln (1 - Ø) ÷ (4.5e-5 x (T+460))] ^ 1/1.5681
Thus, for example, if a wood sample or piece of furniture is kept in an atmosphere
PURDUE UNIVERSITY • Agricultural Experiment Station • Lafayette, Indiana

Research Progress Report 311 July, 1967
Optimum Wood Moisture Content Levels for the Manufacture
Of Furniture in Indiana
C. A. Eckelman and F. E. Goodrick, Department of Forestry and Conservation
Introduction
Wood is a hygroscopic material and as such will tend to adsorb or desorb moisture until it comes into equilibrium with water vapor in the atmosphere. Adsorption and desorption actually occur simultaneously --water molecules are constantly entering and leaving the surface of the wood. But if one phenomenon takes place at a faster rate than the other, a net adsorption or desorption occurs. When a point is reached at which the rate of adsorption just equals the rate of desorption, the moisture in the wood is said to be in equilibrium with that in the surrounding atmosphere. The moisture content level at which equilibrium occurs if referred to as the equilibrium moisture content (emc), where moisture content is expressed as a percentage of the oven-dry weight of the wood. The corresponding relative humidity of the surrounding atmosphere is referred to as the equilibrium relative humidity. Although many exceptions exist, the equilibrium moisture content for given relative humidity conditions is usually taken the same for all species.
The relationship between the equilibrium moisture content of wood and the corresponding relative humidity is usually expressed by means of typical equilibrium moisture content
curves. These curves may be approximated by mathematical expressions of the form
EMC = [ -ln(1 - Ø) ÷ C(T+460)]^1/n
where Ø = the relative vapor pressure
T = degrees F.
C = a constant
n = a constant
For the particular case of T = 70 degrees F this expression may be written as
EMC =[(-ln (1 - Ø)) ÷ .02345] ^ 1/1.5235
or, for any temperature between 32 and 210 degrees F. as
EMC = [ -ln (1 - Ø) ÷ (4.5e-5 x (T+460))] ^ 1/1.5681
Thus, for example, if a wood sample or piece of furniture is kept in an atmosphere
PURDUE UNIVERSITY • Agricultural Experiment Station • Lafayette, Indiana